The present invention relates to a feed supplement for arthropods, in particular for lepidopteran larvae and more particularly for larvae of the common silkworm species known as Bombyx mori. 
More precisely, the present invention relates to a feed supplement capable of inducing the absorption of nutritional molecules from feed by intestinal cells of the lepidopteran larvae, in particular silkworms, as well as a faster growth of the larvae and a remarkable reduction of mortality.
G. C. Rock et al., in xe2x80x9cUtilization of methionine analogues by Argyrotaenia velutinana larvaexe2x80x9d Ann. Entomol. Soc. Am, Vol 66 (no 1) 1973:177-179 discloses that L-methionine ethyl ester may substitute methionine as a sulphur source in the diet of the arthropode A. velutinana. 
Bombyx mori silkworm is known to be used for the production of silk yarn.
The production of silk yarn and its transformation are very important from the economical and commercial point of views, and remarkable efforts have been made to improve production while decreasing costs and making the growth of these larvae faster.
Bombyx mori silkworm larva is strictly monophagous in that it only feeds on, mulberry leaves (Morus alba or nigra). This behavior is a serious drawback and limitation in sericulture, in that mulberry leaves are not available all the year round, they are exposed to atmospheric pollution and to parasites and, when treated with pesticides, they can transmit said substances to the animal.
Furthermore, growing, harvesting and administering mulberry leaves remarkably affect rearing costs.
In order to obviate these drawbacks, artificial diets have been studied and proposed, containing essential nutrients for larval growth in low but still sufficient amounts so as to cause no alterations of the alimentary behavior and of the growth of the larvae, and a certain percentage of mulberry leaf powder for inducing the larvae to accept such feed. Among the essential nutrients, of particular relevance is the protein source. It is, in fact, known that amino acids are the primary source of energy for weight growth of silkworms and for silk biosynthesis and that the subsequent development and fertility depend on the type of the protein source.
Moreover, a number of polyphagous polyhybrids have been selected, capable of accepting artificial diets containing low-cost nutrients, particularly as regards the protein source.
The main drawback of such selection is that the selected hybrids very often produce silk of lower quality than that produced by larvae reared on mulberry leaves. Moreover, for all types of silkworms, the larval life cycle, particularly the fifth instar, during which the feed uptake by the larvae is the highest, is very long and, in many cases, mortality in the first three larval instars is considerable.
Therefore, an object of the present invention is to induce a faster growth of the larvae, increasing the body weight and decreasing the duration of larval instars, in particular of the fifth instar, thereby reducing the whole rearing cycle.
A further object of the present invention is to reduce mortality, particularly in the first three larval instars and to produce last instar larvae and pupae of higher weight than with a normal feeding cycle.
More precisely, the object of the present invention is to provide activating molecules capable of enhancing the assimilation of the ingested protein substances, administered together with natural diet or introduced as a component of artificial diets.
The inventors found that lower alkyl esters of biologically active acids or alpha-amino acids are capable of promoting the absorption of nutritional molecules from diet by the intestinal cells of the larvae, inducing a faster growth and a significant reduction of mortality of the larvae.
According to a first aspect, the present invention provides a feed supplement comprising a low alkyl ester of a biologically active acid or alpha-amino acid and the use thereof for the preparation of both natural and artificial diets for arthropods.
According to a further aspect, the present invention relates to a feed composition comprising a natural or artificial feed for arthropods and a low alkyl ester of a biologically active acid or alpha-amino acid.
According to a further aspect, the present invention relates to a method for inducing a fast growth of lepidopteran larvae, in particular Bombyx mori, increasing their body weight and decreasing the duration of larval instars, thereby reducing the whole rearing cycle, as well as decreasing mortality, which method consists in administering orally said animals with a feed supplement consisting of a low alkyl ester of a biologically active acid or alpha-amino acid.
According to the invention, preferred arthropods are lepidopteran larvae, more particularly larvae of the common silkworm Bombyx mori. 
Low alkyl esters of biologically active acids or of alpha-amino acids are compounds well known in literature. These compounds can be represented by the following general formula: 
in which:
X is hydrogen or NH2;
R1 is a straight or branched alkyl or alkenyl group, containing 3 to 6 carbon atoms, optionally substituted with at least one hydroxy, carboxylic, amino, amido, guanidyl, thio, methylthio, phenyl or imidazolyl group; and
R2 is a group selected from methyl, ethyl and propyl.
Particularly active for the purposes of the present invention are compounds of formula (I) in which X is NH2 and R1 is an alkyl group containing 4 carbon atoms, more preferably those in which X is NH2 and R1 is a branched alkyl group containing 4 carbon atoms.
Examples of compounds of general formula (I) useful as feed supplements according to the present invention are methyl, ethyl and propyl esters of the following acids and alpha-amino acids: 4-methyl-valeric acid, 4-methyl-3-pentenoic acid, 3-methyl-valeric acid, 3-methyl-3-pentenoic acid, L-alanine, L-valine, L-leucine, L-isoleucine, L-methionine, L-threonine, L-aspargine, L-arginine, L-glutamine, L-lysine, L-phenylalanine and L-histidine.
Most preferred compounds are L-leucine and L-methionine methyl and ethyl esters.
The alkyl esters of acids or alpha-amino acids of the present invention can be added to natural or artificial diet, preferably as aqueous solutions. Solutions having concentrations ranging from 0.1 to 2 mM can be used.
The solution is added to the artificial diet or it can be sprayed on the natural diet.
The amount of alkyl ester of a biologically active acid or alpha-amino acid to be added to the diet can range within wide limits, depending on the type of diet and on the species of arthropods. Amounts ranging from 10 to 500 mg/kg of natural or artificial diet can be used.
In vivo tests carried out by the inventors on both polyhybrids and races of Bombyx mori have proved that the addition of an alkyl ester of a biologically active acid or alpha-amino acid to natural and artificial commercial diet gives the following results:
faster growing of the larvae, with an increase in body weight and a decrease in the duration of larval instars, with consequent reduction of at least 24 hours on the whole rearing cycle;
remarkable reduction of mortality in the first three larval instars;
formation of last instar larvae and pupae of higher weight than the control;
production of cocoon shells of weight significantly higher than control, particularly in larvae selected for rearing on artificial diets.
In no cases case alterations of metamorphosis or lower fertility of the adults were evidenced.
Increase in body weight during the fourth and fifth instar for the polyhybrid (28xc3x9715)xc3x97(10xc3x9714) B. mori (panel A) and during the last three instars for the race BG35 (panel B) in silkworms reared on artificial diet (control) and on diet added with leucine methyl ester. Panel C reports the increase in body weight during the last three instars for the polyhybrid (79xc3x9771)xc3x97(55xc3x9762) reared on artificial diet added with methionine methyl ester.